Hydrological and mechanical properties of loess landslides under different vegetation restoration types

Loess landslide is a common geological disaster in northern Shaanxi, which seriously affects people's life and property safety and social and economic development. The research on vegetation restoration types and hydrological and mechanical properties of loess landslides can provide basic data support for landslide stability prediction, and further provide reference for landslide prevention and treatment. In the present study, the loess landslide point of Zhang Zi Gou in Gan Quan County, Yan’an City was taken as the research object. On the basis of the existing natural condition data, the basic physical and mechanical properties and hydrological characteristics were obtained by collecting field landslide soil samples for indoor experimental analysis. The indoor analysis shows that the landslide is mainly distributed in dry land, medium coverage and low coverage grassland, indicating that the surface vegetation coverage can affect the stability of landslide. The worse the vegetation coverage, the more landslides occur. The void ratio and porosity of landslide soil decrease with the increase of dry density. The cohesion of natural soil is obviously higher than that of saturated soil, and the internal friction angle of natural soil is slightly lower than that of saturated soil. In general, due to the influence of water content, the shear strength of natural soil samples is higher than that of saturated soil samples. Therefore, in order to improve the accuracy of prediction and early warning system, it is necessary to consider the response of hydrological and mechanical properties of loess to vegetation restoration. The results provide basic data support for the establishment of loess landslide stability prediction system and provide reference for geological disaster management. Bangladesh J. Bot. 50(3): 795-801, 2021 (September) Special

The Trend of Permeability of Loess in Yili, China, under Freeze–Thaw Cycles and Its Microscopic Mechanism

Loess landslides induced by the freeze–thaw effect frequently occur in Yili, China. Freeze–thaw cycles cause indelible changes in the soil microstructure, affecting its permeability. This study investigated the impacts of freeze–thaw cycles on the permeability of Yili loess using permeability tests on undisturbed (virgin, in situ) and remolded loess samples taken before and after freeze–thaw cycles. Scanning electron microscopy and nuclear magnetic resonance techniques were utilized to investigate the microscopic mechanism of the freeze–thaw process on the loess. Grey relation analysis (GRA) was employed to analyze the correlation between macroscopic permeability and microscopic parameters (maxi. radius, eccentricity, fractal dimension, directional probability entropy, and porosity). The results revealed that the permeability and all the microstructure parameters have roughly shown the same trend: “fluctuation–towards equilibrium–stability”. Firstly, the permeability coefficients of original and remolded loess experienced three and two peaked–trends, respectively, before 30 freeze–thaw cycles. The trends eventually stabilized within 30–60 freeze–thaw cycles. Increased number of freeze–thaw cycles disintegrated large particles in undisturbed loess into medium–sized particles, and particle shapes became more uncomplicated. Medium–sized particles in the remolded loess agglomerated to larger particles with more complex shapes. Furthermore, the overall porosity of the originally undisturbed loess decreased, and large and medium–sized pores transformed into small pores and micropores. In contrast, the overall porosity of remolded loess increased. Finally, the results revealed that permeability coefficients of the undisturbed and remolded loess became closely related with eccentricity and porosity, respectively. This study provides a reference for preventing and governing the loess landslides induced by the freeze–thaw cycles and permeability reduction in construction on loess in seasonally frozen areas in Yili.

Effects of sliding liquefaction on homogeneous loess landslides in western China

Sliding liquefaction is considered to be the cause of high-speed and long-distance sliding of some homogeneous loess landslides in western China. However, there is still a lack of necessary experimental research and analysis on the effects of sliding liquefaction on these landslides. In this work, the effects of sliding liquefaction on irrigation-induced, high-speed and long-distance loess landslides on the South Jingyang Tableland area in China are studied by performing large-scale ring shear tests and using the sled mode. The results are as follows. (1) There are two kinds of long-runout sliding modes of loess landslides on the South Jingyang Tableland: sliding along the terrace surface and sliding within the saturated terrace alluvium, which is associated with sliding liquefaction. Both sliding modes can lead to long-runout sliding. (2) There are some differences in the inclination of the sliding surface between the two sliding modes. Based on the inclination of the sliding surface, the corresponding sliding mode can be distinguished. (3) Under the two sliding modes, the large shear mechanical properties of the two-layer soil composed of loess and alluvial sandy silt show significant differences. The friction between the loess and dry terrace alluvium increases with increasing normal stress and shear rate, while the friction between the loess and saturated terrace alluvium presents the opposite trend. The results show that the sliding distances under different sliding modes present opposite trends with the change in sliding speed. (4) Based on the test results from the ring shear tests and the morphological characteristics of the sliding surface, the sliding mode and sliding distance of a loess landslide can be identified and predicted.

Impact-Induced Liquefaction Mechanism of Sandy Silt at Different Saturations

Landslide-induced liquefaction has received extensive attention from scholars in recent years. In the study of loess landslides in the southern Loess Plateau of Jingyang, some scholars have noted the liquefaction of the near-saturated sandy silt layer that is caused by the impact of loess landslides on the erodible terrace. The impact-induced liquefaction triggered by landslides is probably the reason for the long-runout landslides on the near-horizontal terrace. In order to reveal the mechanism of impact-induced liquefaction, this paper investigates the development of pore pressure and the impact-induced liquefaction of sandy silt under the influence of saturation through laboratory experiments, moisture content tests, and vane shear tests. It has been found that both the total pressure and pore water pressure undergo a transient increase and decrease at the moment of impact on the soil, which takes 40–60 ms to complete and only about 20 ms to arrive at the peak. Moreover, silty sand with a saturation of more than 80° was liquefied under the impact, and the liquefaction occurred in the shallow layer of the soil body. The shear strength of the liquefied part of the soil is reduced to 1.7∼2.8 kPa. Soils with lower saturation did not liquefy. The mechanism of the impact-induced liquefaction can be described as follows: under impact, the water in the soil gradually fills the pores of the soil body as the pore size decreases, and when the contact between the soil particles is completely replaced by pore water, the soil body loses its shear strength and reaches a liquefied state. Soils in the liquefied state have a very high permeability coefficient, and the water inside the soil body migrates upward as the particles settle, resulting in high-moisture content in the upper soil.

Analysis of loess landslide mechanism and numerical simulation stabilization on the Loess Plateau in Central China

Loess landslides have complicated deformation mechanisms. Accurately describing the internal failure deformation of loess landslides and establishing a theoretical method of landslide instability evaluation for the prevention of subsequent landslides have become important topics in western development project construction in China. This paper presents a case study of the Zhonglou Mountain landslide in Shaanxi Province, China. Based on field investigation results, a two-dimensional stability analysis model was constructed using the finite element method. Taking the deformation characteristics of the landslide as the research basis, the distribution laws of the displacement, stress, and shear strain of this landslide were identified with the strength reduction finite element numerical simulation method. Additionally, the safety factor was evaluated under normal and storm conditions. The numerical simulation results show that the horizontal tensile stress of the landslide was mainly distributed in the middle and upper parts of the landslide under normal conditions, while the vertical tensile stress was distributed near the sliding surface. Under heavy rainfall, the sliding force increased, and the anti-sliding force and anti-sliding section decreased; the location of the maximum shear strain shifted down from the middle and upper parts of the landslide body to the area with a shear crack, and the plastic shear strain area expanded along nearly the entire the sliding surface, leading to the occurrence of a landslide. Thus, the use of anti-slide piles to stabilize the landslide was proposed and tested. Monitoring points were arranged along the sliding surface to evaluate the displacement, stress, and strain responses. The on-site observation results agreed with the modeling results. The use of anti-slide piles was demonstrated to be an effective stabilization method for the Zhonglou Mountain landslide.